1. Field of the Invention
The present invention relates to a cover for an air bag used in an automobile and also to a method of producing the air bag cover. More specifically, when an automobile comes into collision, the occupant of the car is flung forwardly by the impact of the collision. As a result, an air bag that is installed in front of the seat is inflated instantaneously with a high-pressure gas. Thus, the inflated air bag serves as a cushion to protect the occupant from dangers of the collision. Such a device is generally known as "air bag safety device".
Thus, the present invention relates to a cover for an air bag used in the air bag safety device and also to a method of producing such an air bag cover.
2. Description of the Prior Art
Recently, an increasing number of air bag safety devices have been used in automobiles in order to reduce the rate of death from traffic accidents. There is a growing tendency to enact legislation for the use of an air bag safety device, and the percentage of automobiles equipped with an air bag safety device has been increasing more and more.
To make sure, a typical air bag safety device will be explained below. The air bag safety device is designed to protect the human body or the like from an impact of collision, and it is provided with an air bag for this purpose. The air bag is orderly folded up in an air bag cover that is attached to the air bag safety device.
The air bag cover has a tear line formed at a proper position thereof so that the air bag cover is readily broken at the tear line by a predetermined internal pressure. If the vehicle collides against something, gas generant is ignited through a certain sensor to inflate the air bag. As the air bag inflates, the tear line is stressed, resulting in the air bag cover being torn at the tear line. Then, the cover opens, and the air bag springs out through the opening of the cover.
The air bag thus sprung out receives the occupant's body to protect him or her from an impact of the collision. Since this chain of operations takes place almost instantaneously, when it is torn at the tear line, the air bag cover is likely to burst into fine fragments. In general, the fine fragments burst out at an extremely high speed. There is therefore a danger that such fine fragments may stick in the occupant's body. For this reason, there has been a demand for an air bag cover designed so that it is prevented as much as possible from bursting into fine fragments when torn at the tear line.
In the meantime, the air bag cover is secured to the body of the air bag safety device by using fasteners, for example, rivets, screws, bolts, etc. When the air bag inflates on collision of the vehicle, the air bag cover accommodating the air bag is torn by the pressure of the inflating air bag, and the air bag springs out to project. At this time, an extraordinarily large force acts on that portion of the air bag cover which is secured to the body of the air bag safety device.
Accordingly, the mounting portion of the air bag cover that is secured to the body of the air bag safety device must have sufficiently high mechanical strength. It is undesirable for the mounting portion to break when the opening portion of the cover is torn, as a matter of course. Under these circumstances, an air bag cover having an integral two-layer structure comprising a soft surface (skin) layer and a rigid core layer has been proposed (see, for example, Japanese Patent Application Laid-Open (KOKAI) 1-202550). The prior art disclosed in this publication has an advantage in that when the air bag cover opens, the core layer is prevented from bursting into fragments by the soft surface layer.
However, the two-layer type air bag cover needs to first form a core layer and then inject a skin layer wide over the whole surface of the core layer to thereby unite them together. It is therefore necessary to prepare separately a first mold cavity for forming the core layer and a second mold cavity for forming the skin layer.
The core layer formed in the first mold cavity is moved to the second mold cavity, and then the skin layer is injected over the whole surface of the core layer. Otherwise, it is necessary to use a split mold and execute a molding process such that the mold is moved after a core layer material has been injected into a cavity for forming a core layer and then a cavity for forming a skin layer is newly prepared and a skin layer material is injected into the cavity. In this case, the injection molded air bag cover is difficult to remove from the mold because of a complicated configuration including undercuts, and an extremely complicated mold structure is needed to cope with such a configuration with undercuts.
In addition, since in the air bag cover having the two-layer structure the area of contact between the core layer and the skin layer is wide, no adequate adhesion is obtained between the core and skin layers with the above-described method, so that the molded product is inferior in the peel strength between the core and skin layers.
With the above-described technical background, the present invention aims at attaining the following objects:
It is an object of the present invention to provide an air bag cover which needs no complicated mold because an opening portion thereof, through which the air bag springs out, has no two-layer structure, and also provide a molding method for producing the air bag cover.
It is another object of the present invention to provide an air bag cover which is designed so that the constituent material is unlikely to scatter when the air bag cover opens, and also provide a molding method for producing the air bag cover.
It is still another object of the present invention to provide an air bag cover wherein a mounting portion thereof, which is secured to the body of an air bag safety device, has sufficiently high mechanical strength, and also provide a molding method for producing the air bag cover.